You are in your car, heading to an intersection. The light turns yellow, so he decides to hit the accelerator. Then you see a police car.

Almost instantly, you know that stepping on the accelerator is a big mistake. But there's a good chance he'll do it anyway, says Susan Courtney, a professor in the Department of Psychological and Brain Sciences at Johns Hopkins University.

That's because one area of ​​your brain recognizes that police car, other areas have already begun to carry out their original plan to accelerate. "Even if you have not started to move your foot, your brain has already started that plan," says Courtney.

And stopping a plan once it's underway requires a great deal of intellectual ability, Courtney and a team of researchers report in the journal Neurona. "It's complicated," he says.

The team monitored the brain activity of 21 people and a monkey when they encountered a situation similar to that of an intersection when the light turns yellow.

Participants were asked to focus on a central point on a screen and wait for a goal to appear elsewhere. Sometimes they were allowed to do the natural thing and change their look to the target when it appeared.

Other times, they had a visual signal to cancel the impulse to change their look. In other words, they had to cancel an action that the brain was already planning to carry out.

The study found that stopping an action required three key areas of the brain to communicate with eight other areas. Previous research had suggested that fewer areas were needed.

The team also discovered that all communication had to occur in about one-tenth of a second when a participant saw the signal not to move the eye. After that, a signal has already been sent to the eye muscles and there is no way to stop it, says Courtney.

This delay is the reason why we experience that horrible and fleeting moment when our brain knows we should not step on the gas, but our foot does it anyway. "If the signal has already been sent, you can see it happen without being able to stop it," says Courtney.

The brain's detention system seems to be involved in much more than simply controlling our bodies. "It's not just about stopping your foot or your eyes, it's about changing your plan about anything," Courtney says.

A function of brain systems that stop an action can help us avoid danger, says Russ Poldrack, a professor of psychology at Stanford University who was not part of the study.

"People now think that some of these same systems are involved in being more cautious when making decisions, taking fewer risks," he says.

In addition, there is growing evidence that these systems are defective in people who have suffered brain damage or who abuse drugs, says Poldrack, adding that years ago, his laboratory did a study of people who take methamphetamine.

"Methamphetamine users worsened by stopping these very simple tasks and their ability to stop themselves is related to the amount of cravings they had for the drug," he says. The longing seems to interfere with the brain's ability to change a plan that it knows is a bad idea.

Ultimately, understanding the brain's arrest system can help us understand the success of our species, says Poldrack.

"The thing that humans do better than any other species is to adapt to the world when things change," he says.

Poldrack agrees with most of what he discovered Neuron although he would like to see a larger study before concluding that all areas of the brain interact in the way that Courtney believes they do.